1. Field of the Invention
The present invention relates to an image projector and an illumination device used for the image projector.
2. Description of the Related Art
Recently, there have been put to practical use various kinds of electronic color imaging systems such as a self-emission color imaging system, a cathod ray tube (CRT) color imaging system and a color imaging system employing a liquid crystal display device as a light valve.
These various kinds of electronic color imaging systems are used for providing people with various kinds of information, for instance, community information in major cities, information for passersby on the street, information in base ball stadiums and race tracks, and for information in the place of an event.
Among the above color imaging systems, there comes to be widely used a color image projector (a projection type color image display apparatus),employing a liquid crystal display device as the light valve, wherein an active matrix liquid crystal display device is employed as a light modulation member.
As the active matrix liquid crystal display device, there are well known two types, a transmission type and a reflection type. In the image projector, priority is given to high luminance and contrast ratio of a displayed image.
The transmission type active matrix liquid crystal display device has a plurality of picture cells being two-dimensionally aligned and light shielding members such as black matrix or black stripes are provided between the cells to enhance picture quality. Cell controlling elements such as thin-film transistors for individually controlling a degree of light passing through the picture cells are provided close to the picture cell. These controlling elements and further provided data signal bus lines and scanning bus lines for driving the transmission active matrix liquid crystal display device are not light transmissive.
Thus, in the transmission type active matrix liquid crystal display device, not all the light beams impinging on the liquid crystal display device are utilized used for displaying an image but a part of them. Therefore, a vignetting (opening) factor defined as a ratio of a quantity of light outputted from a unit picture cell to a quantity of light inputted on its entire picture cell domain is naturally decreased when a number of the picture cells is increased for a given total area of the transmission type active matrix liquid crystal display.
Accordingly, when a high definition picture obtained by increasing the number of picture cells is intended to be displayed with a high luminance, it requires a high intensity of light to illuminate the transmission active matrix liquid crystal display device.
Generally, a liquid crystal used in the liquid crystal display device is not a lyotropic liquid crystal in which a liquid crystal phase appears due to an effect of a solvent, but a thermotropic liquid crystal in which the liquid crystal phase appears in a specific temperature range. The liquid crystal transforms its phase among a crystal phase, a liquid crystal phase and a liquid phase responsive to a temperature. Accordingly, upon irradiation of the incident light beam it is necessary to keep an operative temperature of the liquid crystal display device, which is limited to a certain range.
As mentioned in the foregoing, the vignetting factor is not 100%. Thus, a part of the incident light beam which is not used to display the image, results in a temperature rise of the liquid crystal display device. This fact suggests that it is not always possible or effective to employ a high power source only to obtain a high luminance picture.
Therefore, in the transmission type active matrix crystal liquid display device, there are devised various kinds of countermeasures for increasing the vignetting factor thereof.
On the other hand, in the aforementioned reflection type active matrix liquid crystal display device, there are no light blocking members in the projection light path, such as the thin-film transistors for controlling a quantity of light at every picture cell, the signal bus lines and the scanning bus lines, resulting in the vignetting factor of 100%. This feature is considered to be more advantageous than that of the transmission type display element from the view point of easily obtaining the display image having a high luminance.
Incidentally, as the color image projector employing the light valves, there are two types, a three-plate type color image projector composed of three light valves, each for displaying a primary color image in the additive primaries, and a single-plate type color image projector composed of a light valve with color filters, wherein an image of tri-color primaries is displayed on the light valve driven by a dot or line sequential tri-color signal, and the color filters are disposed so as to produce respective color image of the primaries from the light valve. Each filter has a certain wavelength range corresponding to a certain primary color image.
In the three-plate type color image projector, it requires a tri-color separation optical system and a tri-color composition optical system, inevitably resulting in a largesized projector. On the contrary, the single-plate color image projector has an advantage capable of downsizing the image projector.
Accordingly, there has received much attention the color image display apparatus employing a reflection type active matrix crystal liquid display device as a light valve because of merits of both readily obtaining a high luminance display image and downsizing the apparatus.
However, in order to obtain a high luminance display color image, it is necessary to produce high intensity light beams of primary colors having corresponding wavelength ranges, and each of the light beams having a specific lineally polarized plane. The high intensity light beams should precisely impinge on corresponding tri-color picture cells disposed in a predetermined matrix form on the light valve for spatially modulating the respective incident light beams.
Thus, as the color filter used with the reflection type active matrix liquid crystal display device, it is desirable to employ one capable of performing a thermally stable operation even when the incident light beam impinges thereon with a high intensity.
As such a color filter as satisfying the abovementioned condition, it is preferable to employ a hologram color filter capable of converging the incident light beams on the cells at different angles of diffraction individually for different colors, for instance, as disclosed in the Japanese Patent Laid-open Publication No. 9-189809.
However, in order to cause the hologram color filter to perform a predetermined good diffraction or spectrum-filterring (referred to as diffraction/spectrum-filterring) operation, it is preferable to cause the incident light beams to impinge on the hologram color filter approximately at an incident angle of 60.degree..
As mentioned in the foregoing, when a polarized light beam (S-polarized light beam) oscillating in a direction perpendicular to a plane of incidence with respect to the hologram color filter impinges on the hologram color filter at an incident angle of 60.degree., an oscillation direction of a luminous flux having a cone angle of 0.degree. is perpendicular to the plane of the incidence with respect to the hologram color filter.
However, when an oscillation direction of the luminous flux having a cone angle of more than 0.degree., the oscillation direction of the luminous flux is inclined against that of the luminous flux having the cone angle of 0.degree. because the oscillation direction of the polarized light beam is always perpendicular to a propagation direction of the luminous flux.
Thus, it will be understood that the oscillation direction of the luminous flux having the cone angle other than 0.degree. is no longer an S-polarized light with respect to the hologram color filter.
Therefore, in the image projector employing the liquid crystal display device where the aforementioned incident light impinges on the hologram color filter, projecting a high contrast ratio display image on the screen can not be expected.